For certain applications, especially in the area of therapy, geometric accuracy of magnetic resonance images is important. To measure the geometric accuracy of a magnetic resonance imaging (MRI) scanner, phantoms with MRI visible material in well-defined locations are typically used. For a limited field-of-view (FOV), such as a spherical volume of 200-300 mm in a diameter, such phantoms have existed for a while. However, in the context of radiotherapy, the geometric accuracy needs to be characterized in the whole volume covered by the patient and used in the radiotherapy planning; the skin contour can fill a spherical or cylindrical volume of up to 500 mm in diameter.
For example Stanescu et al Investigation of a 3D system distortion correction method for MR images, Journal of applied clinical medical physics, vol 11(1), 2010 describes a phantom for use in the field of radiation treatment planning. The phantom has a size of 260×260×280 mm3.